Intraspecific variation modifies ecological processes and ecosystem functioning. Still, we know relatively little of how the nature and strength of ecosystem effects caused by intraspecific variation may interact with climate change. We conducted a mesocosm experiment to test if, and to what extent, ocean warming modifies the ecological impacts of intraspecific variation in a predatory fish. The mesocosms consisted of a simplified coastal food web with threespine stickleback (Gasterosteus aculeatus) as the top predator, from a population where two stickleback phenotypes with either complete or incomplete lateral armour plating coexist and display differentiated predation behaviour: The completely plated phenotype often feeds more on invertebrate herbivores compared with the incompletely plated phenotype. Presence of stickleback reduced biomass of arthropod shredders (crustaceans, insect larvae). Warming (+4°C) strengthened this predation, releasing benthic primary producers (diatoms) from top-down control, causing a trophic cascade. This trophic cascade was attributed to one of the plate phenotypes: the completely plated stickleback increased their predation on shredders under warming, while the incompletely plated stickleback instead decreased their predation. Diatom biomass responded accordingly: warming increased diatom biomass in the presence of completely plated stickleback but not when incompletely plated stickleback was present. Our results suggest that different plate phenotypes of threespine stickleback differentially affect lower trophic levels and that warming may exacerbate these cascading effects. These trait-dependent effects on trophic cascades highlight the consequences of intraspecific variation on ecosystem functioning. Read the free Plain Language Summary for this article on the Journal blog.